Sodium Ferrocyanide
What it does
Anti-caking agent in salt.
How it's made
Manufactured.
Sodium ferrocyanide is used in tiny amounts as an anti-caking agent in salt, where it stops the crystals sticking together into hard lumps and keeps the salt pouring freely. Despite the alarming-sounding "cyanide" in its name, the cyanide here is locked tightly to an iron atom in a very stable complex and is not available in the way free cyanide would be.
It is manufactured industrially through a series of chemical reactions that build the ferrocyanide complex around an iron centre, historically starting from hydrogen cyanide or related feedstocks combined with iron salts and then converted to the sodium form. The result is a stable crystalline salt in which the cyanide groups are firmly bound.
The purified salt is added to food salt at extremely low levels — just a few parts per million — because only a trace is needed to change how the crystals grow and prevent caking. Its permitted use is limited to this specific anti-caking role and tightly restricted in amount.
Possible reactions
Safe at permitted levels.